My invention concerns a printing plate of very extended longitudinal dimension (e.g., of the order of meters) capable of printing correspondingly long images on a continuous web of paper or the like with a single impression. More specifically my invention pertains to such a printing plate of the letterpress or relief variety for use on a rotary press of the kind disclosed herein. It is also specifically directed to a method of preparing such an elongate letterpress printing plate.
As is well known, in a rotary press, the printing plate is wrapped around a plate cylinder and prints on the paper against an impression cylinder. Thus the circumferential dimension of the plate cylinder determines the length of the plate, or of the image to be printed at one time. For very long productions, therefore, such as continuous accounting forms for the imprinting of computer outputs, several printing plates have had to be prepared and used one after another on one and the same web of paper. This conventional practice incurs, of course, a substantial waste of time and much printing cost.
I am aware of one known solution to the problem, calling for the use of an endless belt of rubber or the like to be wrapped around the plate cylinder and a guide cylinder spaced therefrom. A plastic letterpress printing plate is bonded to the rubber belt, which thus serves as the backing for the plate. In this case the total length of the rubber backing, not the circumference of the plate cylinder, determines the unit length of productions, so that any long image can be reproduced with a single impression.
This prior art solution has a serious drawback, however. The contiguous plastic printing plate and rubber backing are subjected to shear as they are repeatedly curved over the plate and guide cylinders and flattened along the paths therebetween. Consequently the printing plate easily comes off the backing, which is very thick in comparison with the plate, thus making impossible the accurate reproduction of the original. The known printing method has therefore failed to wind widespread commercial acceptance.